1. Field of the Invention
This invention relates to a molding method and a molding apparatus for a resin long body.
2. Description of the Related Art
Conventionally, for manufacturing a hollow long body with molten resin by injection mold, a molding method and apparatus, in which a multipoint pin gate or a film gate is provided on an end portion of a mold having a shaft inside and heat plastic molten resin is supplied to an inner portion of the mold, are widely used.
Or, molding of the resin long body is, as shown in FIG. 33, conducted as that plural resin injection holes 47 are provided along a longitudinal direction of a mold 41 (longitudinal direction of the long body), and molten resin 43 is injected to a cavity of the mold 41 through the plural injection holes 47. Or, in a case that the molten resin 43 is injected from an injection hole 47 on an end side of the mold 41 as shown in FIG. 34, the injection of the molten resin 43 is conducted with keeping the temperature of the whole mold 41 constant.
As shown in FIG. 32, flowing speed of the molten resin 43 supplied to the inner portion of the mold 41 becomes irregular in the mold 41 by the disposition of a gate 44 and supplying balance of the molten resin. In this case, on a part that the molten resin 43 goes farther, as shown with two-dot broken lines in FIG. 32, a shaft 42 is formed bent by pressure of the molten resin 43, and molded product curves in an opposite direction because the bent shaft 42 rebounds when the molded product is taken out of the mold.
And, as the molded product, rollers used for OA appliances such as a copy machine, a printer, etc. which are tending to correspond to high-speed processing and large-size paper, are required to be long and highly accurate. In a case that the long body as the product is long and thin as described above, injection pressure of the molten resin 43 becomes high because passage for the molten resin 43 in the mold 41 becomes narrow and flowing resistance of the molten resin 43 is high, and the deformation of the shaft 42 is promoted thereby.
Further, in the conventional method shown in FIG. 33, molded product with good dimensional accuracy is not obtained because velocity distribution and confluence are generated in the injected molten resin 43. And, in a case that the molded product has the shaft 42, the shaft 42 is bent by the injection pressure of the molten resin 43, great curvature is generated in the molded product 45 after the resin is solidified (taken out of the mold 41) as shown in FIG. 35, and the product becomes defective with great core deviation.
And, in the conventional method shown in FIG. 34, especially in a case of molding a thin product with small cross-sectional area, a thin long body can not be molded because flowing of the resin stops when the resin solidifies before the cavity is filled. In this conventional method, when the thickness (wall thickness when the product is a pipe) t and the length L are in the relationship of L/t≧100, it is quite difficult to mold the product with high accuracy.
And, when the mold is conducted with the mold 41 of which temperature is kept higher than the melting point of the resin, although the resin is filled to the downstream side in the longitudinal direction of the mold, a recess (concave) 46 is generated on the molded product 45 as shown in FIG. 36 when the resin is cooled and shrunk because of the high temperature of the mold, and the product becomes defective thereby.
And, in the injection mold with the mold 41 shown in FIG. 37, the resin body 48 solidified in the film gate (runner portion) 44 is cut and removed in a separated cutting process by cutting the gate (runner) along the circumference of the resin body 48 with a cutting blade after the molded product 45 is taken out of the mold 41. That is to say, it is difficult to cut the gate effectively in the molding cycle of serial injection moldings.
It is therefore an object of the present invention to provide a molding method and a molding apparatus for resin long body with which a thin long body of resin can be made and molded with high accuracy with excellent productivity.